WO1999018401A1

WO1999018401A1 - Method and device for drying wood

Info

Publication number: WO1999018401A1
Application number: PCT/AT1998/000234
Authority: WO
Inventors: Johann Wolf; Herbert Fuchs
Original assignee: Wolf Systembau Gesellschaft M.B.H.
Priority date: 1997-10-07
Filing date: 1998-10-02
Publication date: 1999-04-15
Also published as: ATA169997A; EP0950163A1; AT408575B

Abstract

According to the invention, wood is dielectrically heated in a vacuum chamber by a high frequency field which penetrates said wood. The vacuum chamber and all of the parts located in said chamber are kept at a higher temperature than that of the wood. Once the atmosphere in the vacuum chamber is saturated with water vapour, said water vapour is condensed in a separate evacuated condensation chamber. The inventive device for carrying out this method has a vacuum chamber (17) wherein electrodes (4) are arranged for applying a high frequency electrical field to the wood to be dried. A heating system (2) is allocated to the wall of the vacuum chamber (17). A condensation chamber (9) is also provided, said condensation chamber being connected to the vacuum chamber (17) by a line (7) which can be blocked off by a valve (8).

Description

"Process and equipment for drying

The invention relates to a method for the dielectric drying of wood, in a vacuum chamber, in which the wood to be dried is exposed to a high-frequency field penetrating the wood to be dried for the dielectric heating of the wood, the vacuum chamber being heated.

The invention further relates to a device with which the method can be carried out.

Various vacuum wood drying plants and associated drying processes are known in the prior art, which can essentially be divided into two types:

a) Those in which the heat necessary for the evaporation and reduction of its water content is supplied to the wood enclosed in a chamber under reduced atmospheric pressure from the outside via the surface of the wood. This is achieved either by turbulent contact with heated gases, heated vapors, but preferably by means of a water vapor / air mixture. Systems are also known in which the required heat is supplied exclusively or additionally by extensive contact with heated plates, occasionally also by radiation from a heat source.

This type of vacuum drying plant is not the subject of the present invention.

b) Processes in which the heat required for the evaporation of the water of the wood batch enclosed in a chamber under reduced pressure is generated directly in the wood by means of a high-frequency field penetrating the wood via the dielectric losses that occur.

The drying processes differ physically to the extent that in the processes described under a) the inside of the

Water in the wood is mainly reduced by diffusion of the water molecules through the outer, warmer layers of wood In contrast, in the dielectric heating processes listed under b) there is mainly a steam flow from the inside of the wood to the outside, which is caused by the vapor pressure difference (Δ p) due to the temperature inside the wood and the atmosphere surrounding the wood with sub-atmospheric pressure .

The advantages of drying wood in an alternating dielectric field are that the often complicated and expensive, as well as energy-consuming, devices for rapidly circulating and renewing the vacuum-steam atmosphere for the purpose of sufficient convective heat transfer to the wood surfaces are eliminated. Another advantage is that the so-called "boarding" of the wood, which can result from the outer wood layer drying out too quickly, is prevented since a constant flow of steam from the warmer interior of the wood to the outside due to its capillary-porous structure the surface takes place.

However, the previously known devices for vacuum drying wood in an alternating dielectric field have some disadvantages.

The electrode surfaces between which the high-frequency alternating field is effective must enclose the wood, that is, for example a stack of wood several meters long and commercially available in width and height, between them. Zones with different field strengths are formed on the electrodes of such a large area at the frequencies customary for such a method, which are in the range between 3 and 30 MHz, depending on the wavelength of the selected working frequency. The field of large-area electrode plates is no longer homogeneous due to interference and the formation of standing waves. Likewise, the dielectric constant of the wood arranged in the high-frequency field is not uniform over the entire volume of the wood due to different moisture distribution and wood density. Further irregularities in the field strength are caused by air gaps and zones of different vapor density between and around the wood.

As a whole, these influences mean that the heating of the wood is not homogeneous over its length and cross section temperature differences of up to 10 ° C can be determined. In vacuum this causes considerable differences in the drying gradient due to the easier mobility of the water molecules and the lower boiling point, whereby an uneven drying progress occurs over the volume of the wood.

Another disadvantage of the known vacuum drying in the alternating dielectric field in the currently known devices is, however, also that the wood is warmer than its surroundings, in particular warmer than the walls of the vacuum chamber, which in some known constructions are additionally cooled by the to condense water vapor escaping the wood. Devices are also known in which cooling registers are fitted within the vacuum chamber. A large proportion of the heat generated in the wood is radiated to the cooler environment over a large area, and is also released by convection via the steam-residual gas atmosphere of the chamber. This means that the energy required to maintain the correct wood temperature is considerably greater than would be required to supplement the heat of vaporization.

The main disadvantage of the currently known methods, however, lies in the fact that as soon as there are desired or unwanted condensation processes within the vacuum chamber, critical climatic differences arise in the chamber, which result in an uneven progress of drying in the chamber due to the temperature differences already present in the wood stack and on the vacuum chamber reinforce.

The object of the present invention is to avoid the disadvantages described. In particular, the invention is based on the object of vacuum drying wood in an alternating dielectric field

("High-frequency vacuum drying") to improve in terms of energy consumption and the drying result.

The method according to the invention should be able to be carried out in a vacuum chamber with dielectric heating of the wood in a high-frequency field with the formation of a homogeneous, conditioning, climate-optimized chamber atmosphere.

According to the invention, this object is first achieved in that all the space in the vacuum chamber in which the wood to be dried is limiting wall parts and all parts of the vacuum chamber arranged in the chamber are kept at a temperature which is higher than the temperature of the material to be dried and that after reaching an atmosphere saturated with water vapor in the vacuum chamber in a separate from the vacuum chamber arranged, evacuated space (condensation chamber) water vapor is condensed.

Preferred and advantageous embodiments of the method of the invention are the subject of the dependent claims.

The invention also relates to a device with which the method according to the invention can be carried out. Such a device for performing the method with a vacuum chamber, with electrodes arranged in the vacuum chamber for applying an electrical high-frequency field to the wood, and with a vacuum pump, which is connected to the interior of the vacuum chamber via a line with a shut-off device, can by one of the Heater assigned to the vacuum chamber and by a condensation chamber which is arranged separately from the vacuum chamber and is connected to the interior of the vacuum chamber via a line which can be shut off by a valve.

The essential feature of the method according to the invention for dielectric drying, in particular for drying wood, in a vacuum chamber in which the wood to be dried is exposed to a high-frequency field for the dielectric heating of the wood which penetrates the wood to be dried, the vacuum chamber being heated, consists in this that all the wall parts delimiting the space in the vacuum chamber in which the wood to be dried is accommodated and parts of the vacuum chamber arranged in the chamber are kept at a temperature which is higher than the temperature of the wood to be dried.

This ensures that water vapor cannot condense on the inner walls of the vacuum chamber and on the parts arranged in the vacuum chamber, except for the condensation on and in the wood to be dried, which is explained below. The condensation of water vapor escaping from the wood and a resulting drying process of the wood located in the vacuum chamber takes place in the invention in an external, with the vacuum chamber Via a line and a shut-off valve connected, also evacuable condensation chamber. If the valve to the condensation chamber is closed, the drying process in the vacuum chamber which is shut off in this way comes to a standstill, since water vapor cannot condense as soon as the temperature-vapor pressure equilibrium in the vacuum chamber corresponding to the respective wood temperature has been established. Drying continues by opening the valve in the line to the condensation chamber.

The knowledge on which the invention is based is that, despite the non-uniform field strength of the dielectric alternating field in the wood, and the associated, uneven heating of the wood and the resulting final moisture differences over the volume of the wood, the differences in temperature and final moisture of the wood result from the inventive use of an optimized chamber climate and can be largely compensated for by a conditioning process taking place at the same time.

In the method of the invention, the object is essentially achieved by reversing the temperature conditions in the

Vacuum chamber and solved in that no condensation devices are provided in the vacuum chamber.

In a departure from the prior art, the wood to be dried in the vacuum chamber forms the coolest of the masses present there during drying.

The method and the device of the invention are explained in more detail below by way of example:

A vacuum high-frequency drying chamber is connected to an externally arranged, also evacuable condensation chamber via a pipe and a valve. The walls of the vacuum chamber and the electrodes provided in it are placed on a heater by means of heaters attached to or in the chamber walls

Brought temperature that is a few degrees (3 to 4 ° C) higher than the previously determined temperature on the wood, which in the majority of cases will be approximately identical to the prevailing ambient temperature. This process takes place with the vacuum chamber closed but still under atmospheric pressure. Higher temperatures of the Chamber walls are not advisable at this stage, since otherwise the outer wood layer would be unnecessarily heated by heat radiation and convection via the enclosed atmospheric air. It is then pumped to the maximum permissible vacuum and the wood is degassed. If the boiling point of the water corresponding to the existing wood temperature is reached, the pumping process is interrupted. The evacuated and shut-off chamber volume fills with the water vapor escaping from the wood, which cannot condense anywhere in the chamber. A vapor pressure equilibrium is therefore quickly established, which corresponds to the vapor pressure curve of the water for the present wood temperature. Example: 20 ° C = 25 mbar, 30 'C = 42 mbar, etc

The amount of water escaping from the wood to form the vapor pressure equilibrium is small and is, for example, about 17.3 g of water for 20 ° C. and about 30.3 g of water per cubic meter of residual volume in the vacuum chamber at 30 ° C.

The wood can then be heated using high frequency and the heating of the chamber walls can continue. The increasing wood temperature serves as a control variable for the temperature of the chamber walls and other device parts contained in the vacuum chamber, which temperature must always be kept somewhat higher.

The condensation chamber is evacuated either before the start of the process or while the wood is heating up, the valve to the vacuum chamber being closed. In the closed system of the vacuum chamber, the valid vapor pressure equilibrium is established with increasing wood temperature. The amount of water that comes from the wood is irrelevant. The lack of condensation always saturated and homogeneous steam atmosphere in the closed vacuum chamber forms a climate that causes a conditioning process.

Since the high-frequency field heats the wood unevenly, the chamber wall is at a higher temperature and no condensate can be formed there, but the vapor pressure in a closed system always tries to adjust itself to the lowest temperature, both moisture and heat transfer takes place. port to those zones in the wood that have a lower temperature exhibit .

It is a peculiarity of the dielectric heating of wood that zones with greater wood moisture absorb more energy from the high-frequency field and are therefore better heated. Depending on their higher temperature, these zones give off more steam pressure and, in the absence of other options, the increased steam pressure is reduced again at points with a lower temperature. In the method of the invention, this is only possible on the cooler wooden parts within the vacuum chamber. When water vapor condenses, the heat of vaporization contained therein is released again. This conditioning process with regard to moisture balance and temperature compensation in the wood also takes place during the entire drying process.

The actual drying process is initiated by opening the valve to the condensation chamber, which is also under vacuum but is unheated and possibly cooled, at an already high temperature in the vacuum chamber and in the wood. The respective opening cross-section of the valve determines the amount of water vapor that escapes from the chamber climate, which is evenly saturated with vapor pressure, and is intended for condensation.

The water condensing in the condensation chamber, which is removed from the homogeneous, conditioning climate in the vacuum chamber, lowers its vapor pressure. The latter was in equilibrium with the existing wood temperature. This state of equilibrium is restored by the escape of water vapor from the wood to be dried, so that the wood loses moisture constantly and in a controllable manner, ie "dries".

The above explanations show that the evacuable condensation chamber, which is arranged separately from the vacuum chamber in which the wood to be dried is located, is essential for the method according to the invention to be carried out successfully. The reason for this is that the vacuum chamber, which is kept at a higher temperature, does not permit condensation processes in the vacuum chamber itself. As a result, the water removal required for drying by steam condensation requires an external evacuated condensation chamber, which is arranged separately from the vacuum chamber, so that the water vapor escaping from the wood condenses and thus that in the vacuum wood in the chamber can dry.

A device according to the invention, in which the method of the invention can be carried out, is explained in more detail by way of example with reference to the drawings. 1 shows a section of a vacuum chamber in which the method according to the invention can be carried out, FIG. 2 shows a modified embodiment, and FIG. 3 shows a temperature-vapor pressure diagram for water between 0 ° C. and 100 ° C.

A vacuum chamber 17 with a rectangular cross-sectional shape in FIG. 1 has walls 1 to which heating pipes 2 are attached on the outside. The heating tubes 2 are charged with a liquid or gaseous heat transfer medium. Electric heaters can also be used.

The vacuum chamber 17 is surrounded by thermal insulation 3. In the vacuum chamber 17, two surface electrodes 4 are arranged running in the longitudinal direction, each of which is connected to a high-frequency generator 6 via a vacuum-tight high-frequency feed 5.

A heated line 7 with shut-off valve 8 connects the vacuum chamber 17 to an evacuable condensation chamber 9. The condensation chamber 9 is equipped on the outside with cooling fins 10 for convection cooling through the surroundings and on the inside with cooling coils 11, which are fed with a cooling medium by a cooling unit 12.

A valve 13 serves to ventilate the condensation chamber 9 and a valve 14 to remove the condensate (water).

The vacuum chamber 17 is connected to a vacuum pump 18 via a heatable line 15, in which a valve 16 is provided.

Between the electrodes 4 is the wood 19 to be dried, which in FIG. 1 is a stack running in the longitudinal direction of the chamber (perpendicular to the image plane).

Fig. 2 shows a cylindrical vacuum chamber 17 in a double-walled design, the heating of the vacuum chamber 17 with an over a supply line 27 is supplied to a cavity 29 and discharged via a discharge line 28. In the example in FIG. 2, the externally arranged, evacuable condensation chamber 9 is only equipped with cooling fins 10 arranged on the outside for the purpose of convection cooling by the surroundings.

The condensation chamber 9 can (as in FIG. 1) be evacuated separately from the vacuum chamber 17 via a suction line 20 in which and a valve 21 is provided.

3 graphically shows the vapor pressure of water at different temperatures and the amount of water contained in a respective temperature-vapor pressure equilibrium per cubic meter of volume.

In addition to the drying technology advantages of the homogeneous, almost almost saturated chamber climate in the vacuum chamber 17 with its conditioning, temperature and moisture balancing properties, the method according to the invention also has the advantage of considerable energy savings. As soon as the walls of the vacuum chamber 17 have been heated to their working temperature, a further supply of heat is only necessary to the extent that it corresponds to the slight but inevitable heat loss through the insulation 3. However, the wood to be dried can only suffer thermal "losses" due to the "reversed temperature conditions" in the vacuum chamber 17 (wall warmer than wood) due to water evaporation. The energy to be tracked by the high frequency or dielectric heating is at best limited to the addition of the theoretically necessary heat of vaporization, corresponding to the amount of water withdrawn via the condenser from the homogeneous, optimized chamber climate

Time unit. This energy requirement is approximately 540 kcal / kg water, corresponding to approximately 0.626 kWh per kg water.

If there is only a small amount of residual moisture left in the wood at the end of the drying process, there is more to remove

Amount of water to be used per kilogram of water is slightly higher than previously stated.

In summary, a preferred exemplary embodiment of the method can be described as follows: For drying, wood is introduced into a chamber 17 which can be evacuated by a vacuum pump 18 via a line 15 with a shut-off valve 16 and which is connected to a condenser 11 via a further line 7 and a valve 8 with an evacuable condensation chamber 9. In the vacuum chamber 17, two electrodes 4 are provided, via which a high-frequency field can be applied to the wood 19 to be dried in order to heat the wood 19. Furthermore, the walls 1 of the vacuum chamber 17 and the lines 17, 15 are equipped with a heater 2.

The procedure for drying wood is as follows: The vacuum chamber 17 is loaded with the wood 19 to be dried. Then, with the valve 8 closed, the vacuum chamber 17 is evacuated to the condensation chamber 9 by the vacuum pump 18, wherein the condensation chamber 9 can also be evacuated via a line 20. After a vacuum is reached at which water begins to evaporate in the wood 19 to be dried at the given temperature, the valve 16 to the vacuum pump 10 is closed and heating is started by both the heating device 2 assigned to the walls 1 of the vacuum chamber 17 and the dielectric heater with the electrodes 4 is put into operation. Care is taken that the wood 19 to be dried is always cooler than the other parts of the installation, in particular the walls 1 of the vacuum chamber 17. The heating can be carried out to any process temperature above the boiling point of water at the given vacuum. As soon as the desired process temperature is reached, the valve 8 to the condensation chamber 9, which has been previously evacuated, is opened to the desired opening cross section. Characterized in that there is a lower water vapor pressure in the condensation chamber 9, water vapor diffuses from the vacuum chamber 17 into the condensation chamber 9, so that in the

Vacuum chamber 17 can leak further water vapor from the wood. There is therefore a higher water vapor pressure in the vacuum chamber 17 than in the condensation chamber 9 during drying, so that water vapor flows from the vacuum chamber 17, in which the wood 19 to be dried is received, into the condensation chamber 9. The speed of drying can be regulated by setting the valve 8 to the desired opening cross section. An advantage of the method is that no water can condense on system parts within the vacuum chamber 17, since these are always at a temperature which is higher than the temperature of the wood to be dried. zes, so that drying too quickly with the known disadvantages (shrinkage, cracks, etc.) is prevented.

Claims

Claims:

1. Process for the dielectric drying of wood, in a vacuum chamber in which the wood to be dried is exposed to a high-frequency field penetrating the wood to be dried for dielectric heating of the wood, the vacuum chamber being heated, characterized in that all the space in the vacuum chamber in which the wood to be dried is received, wall parts delimiting parts of the vacuum chamber arranged in the room are kept at a temperature which is higher than the temperature of the wood to be dried and that after reaching an atmosphere saturated with water vapor in water vapor is condensed in the vacuum chamber in an evacuated space which is separate from the vacuum chamber.

2. The method according to claim 1, characterized in that the water vapor extracted from the saturated chamber atmosphere is condensed in the evacuated space (condensation chamber) separated from the vacuum chamber by cooling.

3. The method according to claim 1, characterized in that the water vapor extracted from the saturated chamber atmosphere is condensed in the evacuated space separated from the vacuum chamber by water vapor being bound to a hygroscopic substance.

4. The method according to claim 2 or 3, characterized in that when condensing water vapor by cooling water vapor or by binding water vapor to the hygroscopic substance heat is used to heat the vacuum chamber.

5. The method according to any one of claims 1 to 4, characterized in that the speed at which the drying of

Wood is carried out by changing the free flow cross-section between the vacuum chamber and the evacuated space in which the water vapor extracted from the saturated chamber climate is condensed to the desired value.

6. Device for performing the method according to one of claims 1 to 5, with a vacuum chamber (17), with electrodes (4) arranged in the vacuum chamber (17) for applying an electrical high-frequency field to the wood to be dried (19), and with a vacuum pump (18) which is connected to the interior of the vacuum chamber (17) via a line (15) with a shut-off device (16), characterized by a heater (2, 29) assigned to the vacuum chamber (17) and by one of the Vacuum chamber (17) separately arranged condensation chamber (9) which is connected to the interior of the vacuum chamber (17) via a line (7) which can be shut off by a valve (8).

7. Device according to claim 6, characterized in that a cooling coil (11) is provided in the condensation chamber (9).

8. Device according to claim 6 or 7, characterized in that the condensation chamber (9) is provided on the outside with ribs (10) to enlarge their heat-radiating, outer surface.